Winds at departure shape seasonal patterns of nocturnal bird migration over the North Sea

Abstract

On their migratory journeys, terrestrial birds can come across large inhospitable areas with limited opportunities to rest and refuel. Flight over these areas poses a risk especially when wind conditions en route are adverse, in which case inhospitable areas can act as an ecological barrier for terrestrial migrants. Thus, within the east-Atlantic flyway, the North Sea can function as an ecological barrier. The main aim of this study was to shed light on seasonal patterns of bird migration in the southern North Sea and determine whether departure decisions on nights of intense migration were related to increased wind assistance. We measured migration characteristics with a radar that was located 18 km off the NW Dutch coast and used simulation models to infer potential departure locations of birds on nights with intense nocturnal bird migration. We calculated headings, track directions, airspeeds, groundspeeds on weak and intense migration nights in both seasons and compared speeds between seasons. Moreover, we tested if departure decisions on intense migration nights were associated with supportive winds. Our results reveal that on the intense migration nights in spring, the mean heading was towards E, and birds departed predominantly from the UK. On intense migration nights in autumn, the majority of birds departed from Denmark, Germany and north of the Netherlands with the mean heading towards SW. Prevailing winds from WSW at departure were supportive of a direct crossing of the North Sea in spring. However, in autumn winds were generally not supportive, which is why many birds exploited positive wind assistance which occurred on intense migration nights. This implies that the seasonal wind regimes over the North Sea alter its migratory dynamics which is reflected in headings, timing and intensity of migration.

The interplay of wind and uplift facilitates over-water flight in facultative soaring birds

“…We found that, in all flyways that we studied, facultative soaring birds were more selective for wind support than uplift when flying over the open sea. This confirms previous findings that birds aim at maximizing wind support, probably to reduce the time and energy required to complete their journeys [18,[21][22][23][24][44][45][46]. Our model also showed a positive effect of the interaction between wind support and ∆T.…”

“…This confirms previous findings that birds aim at maximizing wind support, probably to reduce the time and energy required to complete their journeys. [18,[21][22][23][24][44][45][46]. Our models also showed a positive effect of the interaction between wind support and ∆T.…”

Associations of Synoptic Weather Conditions With Nocturnal Bird Migration Over the North Sea

“…The blue circle indicates the most probable departure area of the migrants on nights of high migration intensity in spring. The red circle indicates the most probable departure area of the migrants on nights of high migration intensity in autumn outside of the Netherlands according to Bradarić et al (2020).…”

“…Twice a year large flows of terrestrial birds migrate over the North Sea during the night (Lack, 1963a,b;Hüppop et al, 2006;Hüppop and Hüppop, 2011;Shamoun-Baranes and van Gasteren, 2011). The crossing mainly occurs between the lowlands of Netherlands and Germany and Norway (cca 540 km), Denmark and Netherlands (cca 300 km) and between United Kingdom and Netherlands (cca 200 km) (Lack, 1959;Shamoun-Baranes and van Gasteren, 2011;Bradarić et al, 2020). To cross such distances, a bird with the average airspeed of 16 m/s needs between 3.5 and 10 h in still air.…”

“…Thus, we describe the synoptic scale conditions in relation to potential departure areas on nights of high migration intensity. A back-trajectory analysis of migration using the same radar dataset (Bradarić et al, 2020) showed that the most probable departure areas of the birds on nights of high migration intensity observed at the radar site in spring is southern United Kingdom (area in blue circle in Figure 1) and in autumn is Southern Scandinavia, the north coast of Netherlands and Germany (area in red circle in Figure 1). We discuss similarities and differences between spring and autumn in synoptic scale conditions that support or potentially inhibit migration over the southern North Sea.…”